This invention relates to electroluminescent (EL) lamps and, in particular, to an EL lamp having an overprint layer including light scattering particles mixed in with the cascading dyes or phosphors.
An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer includes a phosphor powder or there is a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder emits light in the presence of a strong electric field, using very little current. An EL lamp requires high voltage, alternating current but consumes very little power.
EL phosphor particles are zinc sulfide-based materials, typically including one or more compounds such as copper sulfide (Cu2S), zinc selenide (ZnSe), and cadmium sulfide (CdS) in solid solution within the zinc sulfide crystal structure or as second phases or domains within the particle structure. EL phosphors typically contain moderate amounts of other materials such as dopants, e.g., bromine, chlorine, manganese, silver, etc., as color centers, as activators, or to modify defects in the particle lattice to modify properties of the phosphor as desired. A copper-activated zinc sulfide phosphor produces blue and green light under an applied electric field and a copper/manganese-activated zinc sulfide produces orange light under an applied electric field. Together, the phosphors produce white light under an applied electric field.
Because EL lamps provide uniform luminance and consume very little power, there is a great demand for EL lamps in displays. There is also a great demand for a variety of colors, which is difficult to meet from a limited number of phosphors. The color of a phosphor is a quantum mechanical phenomenon that, by definition, does not provide a continuous spectrum of colors. Thus, EL lamps produce light having a limited spectrum with pronounced peaks. Phosphors emitting different colors can be mixed and a particular spectrum or color is obtained by enclosing a designated point on a CIE [Commission Internationale de l""Eclairage] chromaticity diagram. The available phosphors must define an area that encloses the designated point or area.
It has long been known in the art to xe2x80x9ccascadexe2x80x9d phosphors, i.e. to use the light emitted by one phosphor to stimulate another phosphor or other material to emit light at a longer wavelength; e.g. see U.S. Pat. No. 3,050,655 (Goldberg et al.). It has also long been known to use dyes as the cascading material; e.g. see U.S. Pat. No. 3,052,810 (Mash). It is also known to doubly cascade phosphors. U.S. Pat. No. 6,023,371 discloses an EL lamp that emits blue light coated with a layer containing fluorescent dye and fluorescent pigment. In one example, the pigment absorbs blue light and emits green light, while the dye absorbs green light and emits red light.
Mixing different phosphors, cascading phosphors, and filtering are three of several techniques known in the art for obtaining colors other than the strongest emission band of a particular phosphor. Cascading phosphors and filtering absorb light and therein lies a problem. The net amount of light emitted by an EL lamp depends upon how much light is generated initially, how much is absorbed by cascading materials, and how efficiently the cascading materials convert light to longer wavelengths. Often, a great deal of dye is necessary to produce the desired color.
The amount of dye in an ink affects several aspects of making an EL lamp. Often, the amount dye necessary to produce a desired color absorbs too much light and the lamp is too dim for commercial success. Also, some dyes are relatively expensive, making the cost of some lamps prohibitive. Finally, the amount of dye affects print quality. Inks containing less dye can be printed through a finer mesh than the same ink more heavily loaded with dye. Being able to use less dye or less phosphor, or printing with fewer passes to deposit an effective amount of material, would also benefit the construction of existing types of lamps.
U.S. Pat. No. 3,248,588 (Blazek et al.) discloses using a cascading dye as an xe2x80x9cunderprint,xe2x80x9d i.e. between the phosphor layer and the rear electrode. The patent further discloses adding barium titanate to the dye layer to act as a reflective background and increase brightness. Such a layer as an overprint would be substantially opaque. U.S. Pat. No. 6,225,741 (Nakamura et al.) discloses using barium titanate (BaTiO3) or titania (TiO2) in an organic polymer layer as a separate reflecting layer between the phosphor layer and the rear electrode.
In view of the foregoing, it is therefore an object of the invention to provide an EL lamp that uses cascading phosphor or dye more efficiently than in the prior art.
Another object of the invention is to provide an EL lamp in more colors than were previously available.
A further object of the invention is to provide an EL lamp using cascading materials that is less expensive than lamps using the same materials and constructed in accordance with the prior art.
Another object of the invention is to increase the brightness of EL lamps using cascading materials.
A further object of the invention is to improve the print quality of inks containing cascading material.
Another object of the invention is to be able to print an effective amount of material in fewer passes than in the prior art.
The foregoing objects are achieved in this invention wherein light scattering particles are added to the ink containing the cascading material. The particles and cascading material are then printed in the same layer. Light entering the cascading layer is scattered, re-entering the cascading material thereby increasing the effectiveness of the cascading material and enabling one to use less material. Because less cascading material is used, the cost of the EL lamp is reduced and cascading efficiency is increased. The preferred light scattering particle is titania.